This invention relates to radio-frequency (RF) transmission and more particularly to digital RF transmission.
A conventional prior art digital transmitter system, as shown in FIG. 1, includes a digital baseband signal (11), typically a multibit narrowband signal at the Nyquist sampling frequency in the range of 1 MHz. This is then converted to an analog baseband signal 13 using a narrow band digital-to-analog converter (DAC), 12. The baseband analog signal 1.3 is then fed to an analog mixer 14 to which is fed the output 15 of an analog local oscillator (LO) 16. The output of mixer 14 is then fed to an analog bandpass filter 17 which is used to eliminate undesired mixer products. The resulting analog RF signal, 18, is amplified by a conventional linear amplifier 19 and fed to a transmission antenna 110 for broadcasting.
An alternative prior art system shown in FIG. 2 has been gaining attention in recent years. In this system the signal may be maintained in digital form until much closer to the antenna by employing a concept which is termed “Software Radio” or “Software-Defined Radio” (SDR). Here the digital baseband signal (11) is first upsampled using a fast digital interpolation filter 21 to produce an upsampled signals 22. The up-sampled signals 22 are then multiplied by means of a digital multiplier 23 which is supplied with sampling signals 24 generated by a digital Local Oscillator 25 (operating at a frequency of approximately 1 GHz) to generate a multi-bit digital-RF signal 26. Thus, a digital local oscillator and a digital up converter are used to generate a multibit digital RF signal, 26. This Nyquist-rate multi-bit signal 26 may be converted to an even faster oversampled single-bit digital signal with the same in-band dynamic range, using an “Oversampling Code Converter” (OCC), 27. The OCC 27 may be comprised of a digital delta-sigma modulator, or alternatively a “staggered thermometer code” circuit as described in U.S. Pat. No. 6,781,435. The oversampled bitstream 28 at the output of OCC 27, identified in FIG. 2 as a single bit pulse width modulated (PWM) oversampled digital RF signal, can then be passed through an analog bandpass filter 29 to create a broadband analog RF signal 210, which can then be amplified via a power amplifier such as linear amplifier 19 for transmission to a transmitting antenna such as 110.
While the architecture for a digital-RF transmitter shown in FIG. 2 has been discussed in the prior art, to Applicants' knowledge it has never actually been implemented for a broadband RF signal, because it requires an oversampling code converter (e.g., OCC 27) which has to operate faster than can be achieved by existing circuit technology. Thus, the digital-RF approach is difficult or impossible to achieve with conventional technology, given the very fast multi-GHz sampling and digital processing rates required.
Another problem with processing the multibit digital RF signals 26 (and/or the bit stream out of OCC 27) is that these signals, especially if generated using Josephson junction (JJ) based circuits, are of very low amplitude and need to be greatly amplified to increase their voltage/current (power) level before application to an antenna for transmission. One approach, shown in FIG. 3A, is to take the low power analog RF signal 210 from bandpass filter 29 (see FIG. 2) and feed it to a high-quality linear analog amplifier 39. But this mode of power amplification is power-inefficient if linearity is to be maintained, and can introduce noise into the signal.
An alternative approach, as shown in FIG. 3B, is to maintain the signal in a single-bit digital format. Here the output 28 of the OCC is applied to a single switching amplifier 311 whose output swings quickly between the voltage rails of the power supply, e.g., an amplifier which may be operated as a class S or class D amplifier. If the amplifier switching is fast enough, this will reproduce the input PWM stream with larger amplitude, with good power efficiency and without distortion. However, although the switching scheme of FIG. 3B is more efficient than the scheme of FIG. 3A, it requires circuits that need to switch faster than is available using existing amplifier technology. That is, there is no known single amplifier fast enough to perform this function, at the frequencies of interest.
Thus, although the concepts of FIGS. 3A and 3B have been discussed (see, for example, FIG. 8 in P. Asbeck et al., “Digital Signal Processing up to Microwave Frequencies”, IEEE Trans. MTT vol 50, no 3 pp. 900-909, 2002), to Applicants' knowledge, they have never been implemented for a broadband RF signal, since the required switching speeds are greater than can be obtained using conventional technology.
Accordingly, a problem exists in processing the signals at the high frequencies (e.g., gigahertz range) of interest. This problem is resolved in circuits and systems embodying the invention.